Interior flat latex paints constitute a significant segment of the paint market. Interior flat aqueous based paints, used for painting walls and ceilings, typically contain a film forming polymeric binder system, high concentrations of opacifying pigments and other additives, such as for example, extenders, thickeners, coalescents and rheology modifiers. The pigment provides the paint with certain desired properties, such as for example hiding, sheen, and color, while the polymeric binder provides the paint with its final film integrity.
Conventional polymeric binder systems used in interior flat latex paints are soft polymers which are formulated with water and coalescing cosolvents. After the paint is applied to the substrate surface, the coalescing cosolvent, referred to as "coalescent", and water evaporate from the paint, and the polymeric binder forms a film containing the pigment and non-volatile additives. The ability of the polymeric binder to form a useful film is critical for the final performance of the paint.
The hardness of the polymeric binder is dependent upon the glass transition temperature (referred to hereinafter as the "T.sub.g ") of the polymer. A polymer will not form a film if the ambient temperature which the paint is exposed to upon application to the substrate is lower than the T.sub.g of the polymeric binder. Therefore, the lower the T.sub.g of the polymer, the softer the polymer, and the lower the minimum film formation temperature at which the polymer will form a film upon drying. The ability of a polymeric binder to form a film upon drying is critical because it dictates the minimum temperature at which the paint can be applied to a substrate surface. If the ambient temperature at which the paint is be applied to a substrate is lower than the T.sub.g of the polymeric binder, the ability of the paint to form a useful film upon drying is reduced or possibly eliminated. The minimum film formation temperature and T.sub.g of the polymeric binder are therefore very important when formulating an aqueous latex based paint for use in low temperature environments, such as for example, new construction.
Coalescents are known to be useful for lowering the minimum film formation temperature of polymeric based coatings. The coalescent must be volatile at the temperatures at which the paint is to be applied and soluble in the polymeric binder. Because of the solubility of the coalescent in the polymer, the coalescent acts to effectively lower the apparent T.sub.g of the polymer. The coalescent evaporates from the paint upon drying eventually producing a film of hardness similar to the original glass transition temperature of the polymer. The coalescents are typically used in interior flat paints at concentrations less than about 10% based on the weight of the binder, and more typically on the order of about 3% to 7% based on the weight of binder.
Abrasive scrub resistance is an important property for paints, particularly for latex or aqueous based polymeric paints used on interior surfaces. The ability of a paint film to withstand abrasive scrubbing is a function of the concentration and type of pigments and extenders employed in the formulation, as well as the hardness and particle size of the polymeric binder. Decreasing the pigment volume concentration (referred to hereinafter as "PVC") of a paint formulation can be employed to improve the scrub resistance of the paint. However, this approach is inefficient for two reasons. First, it is more costly due to the increased binder required to replace the relatively less expensive extenders which have been removed. Second, the hiding ability of the coating may be decreased, particularly in high PVC paints.
The softer the paint film is, the lower its abrasive scrub resistance will be. However, increasing the polymer hardness reduces the film formation properties and hence the application window for the formulated paint. This invention employs a polymer with increased hardness yet without reducing the film formation properties.
The particle size of the polymeric binder also has an effect on the scrub resistance of the paint film. Increasing the particle size of the polymer generally decreases the abrasive scrub resistance of the paint, while reducing the particle size of the polymeric binder can improve the abrasive scrub resistance. Reducing the particle size of the polymeric binder, however, typically requires increasing the concentration of surfactants used in conducting the emulsion polymerization process used to make the polymer, with an attendant increase in the surfactant concentration in the paint formulation. Increasing the concentration of surfactants in the paint formulation, however, reduces the abrasive scrub resistance of the paint because surfactants increase the water sensitivity of the paint film.